The present invention relates to a method for manufacturing tooling for and aerospace parts exposed to elevated temperature conditions in oxidative atmospheres.
Superplastic forming/diffusion bonding (SPF/DB) is a process used to manufacture such items as turbine blades, engine nozzle fairings, titanium sheet metal components and other aerospace components. The finished details of such items are quite complex and intricate in their shape and design. The SPF/DB process may be generally described as a process wherein individual metal components are heated to a certain critical temperature at which they are sufficiently pliable and can be elongated or otherwise formed or molded and joined together, yet will be irreversibly set without structural damage, instability or "necking". It is a convenient alternative for mass producing complex multifacetted metal parts or substrates which would otherwise require their various individual components or subparts to be assembled by cold bending and joined with numerous fasteners. The machinery for this process requires a number of intricate parts, such as machined tool inserts for dimensional accuracy. The base metal of which these machined tool inserts are composed must withstand repeated cycles of temperature elevation ranging from room temperature to over 1600.degree. F., and because of the ambient conditions, are exposed to oxidative conditions which lead to deleterious scaling.
In the past, the tooling inserts for the SPF/DB have been manufactured from stainless steel alloys. Although the stainless steel has resistance to oxidation, when subjected to repeated thermal cycles undesirable spalling and scaling results. It also has had its drawbacks with regard to poor diffusion bonding and its use necessitates costly rework in fabricating SPF/DB materials. This poor diffusion bonding may in part result from the regidity of stainless steel.
A potentially more ductile material of composition for the tool inserts would be a titanium alloy and/or titanium itself. Titanium and its alloys are known to exhibit sufficient flexibility to allow the desired bonding effect. This would be extremely beneficial in manufacturing SPF/DB tooling inserts because the inserts could be fabricated from a thinner block than most steels and hot formed to the required curvatures for the inserts, thus resulting in substantially reduced fabrication costs and far less reworking of materials.
However, in the past, titanium alloy inserts have exhibited a tendency to undergo oxidative degradation and scaling at the elevated temperatures required for SPF/DB. Unless the oxidative scaling is removed after each SPF/DB cycle, subsequent parts may be defective. The scale removal step has been very labor intensive and eventually, the insert may be scrapped if excessive scale removal leads to reduction in the thickness of the tool insert below the required tolerances.
Accordingly, a method for manufacturing the SPF/DB tooling inserts from titanium or titanium alloy without the undesirable oxidative scaling would be a significant advancement in the art. Such a method would also enable the effective use of titanium alloys in a number of aerospace parts, including for example, aircraft turbine blades, access doors, missile details, and other fabricated sheet metal parts, which would be far more effectively manufactured from titanium alloys if the oxidative scaling which occurs at excessively high temperatures could be overcome.